In cellular networks for wireless communication, interference often occurs in a cell caused by signals transmitted to or from User Equipments. UEs in nearby located cells, which is a well-known problem. In such a network, the available radio bandwidth is limited and in order to provide capacity for communications in the network having multiple cells, resources pertaining to radio bandwidth must be reused in cells at a sufficient mutual distance so as not to disturb communication for one another by interference. In this context, cells that are located near a serving cell are generally referred to as “neighbouring cells” and this term will be used here in the sense that transmissions in neighbouring cells may potentially disturb transmissions in the serving cell, and vice versa, thus causing interference. It should be noted that a neighbouring cell is not necessarily located right next to the serving cell but may be located one or more cells away, still causing interference.
FIG. 1 illustrates an example when interference occurs between two neighbouring cells A and B having radio coverage provided by a first base station 100A and a second base station 100B, respectively. In cell A, base station 100A sends data signals x to be received by a UE 102 in cell A. The figure also illustrates that the second base station 100B sends data signals y to be received by another UE 104 in cell B using downlink bandwidth resources that coincide with those used for the UE 102, and signal y may therefore be interfered by the downlink transmission signal x from base station 100A when received by the UE 104, indicated by an interfering signal x′.
This disclosure is relevant for cellular networks using any of the following radio access technologies: Orthogonal Frequency Division Multiplexing (OFDM), Single Carrier-Frequency Division Multiple Access (SC-FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Time Division Multiplex (TDM), and Frequency Division Multiplex (FDM). Further, resources pertaining to radio bandwidth will be referred to as “bandwidth resources” for short. Typically, bandwidth resources can be defined by a combination of frequency and time period. In systems of Long Term Evolution, LTE, the bandwidth resources are known as Physical Resource Blocks, PRBs, defined by frequency and time period, and in the following text bandwidth resources can be understood as PRBs when applied in an LTE context.
In recent years, the need for capacity in cellular networks has increased significantly as more users have become active, but also since more advanced and demanding services and UEs have been introduced on the market. Packet-switched communication is used extensively in modern networks such as LTE networks, which enables efficient use of the limited bandwidth resources since such resources are only occupied basically when there is data to send. Consequently, bandwidth resources are scheduled to UEs whenever needed for data transmission. For example, delay-sensitive services such as Voice over Internet Protocol, VoIP, put strict requirements on the scheduling process to cause a minimum of delays and allow playout of the data virtually in real-time. The quality of a voice call is basically determined by sound quality and delay which both may suffer from too much interference on transmitted voice data and control messages.
When a UE has data to send, e.g. speech data, it can send a so-called Scheduling Request, SR, to its serving base station which then responds by granting certain bandwidth resources for the UE to use for uplink transmission of the data, which is signalled from the base station in a so-called Scheduling Grant. On the other hand, when there is data to send from the base station to the UE, downlink bandwidth resources are allocated for the UE and a scheduling message, e.g. called Scheduling Assignment, is sent to the UE on a control channel, e.g. the so-called Physical Downlink Control Channel, PDCCH, ordering the UE to start receiving data on the allocated bandwidth resources defined by frequency and time. In either case, it is a requirement particularly for delay-sensitive services that the communication of data can start within a prescribed time limit, or “deadline”, so as to achieve acceptable Quality of Service, QoS. If the time limit has expired for some data before transmission, that data may have to be discarded since it has become too “old” for playout which naturally could deteriorate the QoS.
In order to reduce battery consumption, UEs can enter so-called Discontinuous Reception, DRX, mode when being “asleep”, i.e. when not receiving messages or data. The DRX mode involves passive and active periods according to a DRX scheme such that the UE is asleep during the passive periods and is turned on at regular intervals in the active periods e.g. for monitoring a prescribed downlink control channel, such as the PDCCH. This gives the base station an opportunity to tell the UE to start receiving data on a particular bandwidth resource that has been allocated and reserved for the UE when there is pending data to be sent from the base station to the UE.
FIG. 2 illustrates schematically such a DRX scheme with a repeated DRX cycle 200 comprised of a relatively short active period 200a when the UE wakes up and can monitor the above channel, and a longer passive period 200b when the UE is asleep and basically turned off. The active period 200a is sometimes referred to as the “On_Duration_Timer” which is typically one Transmission Time Interval, TTI, of 1 millisecond when the UE is active and able to receive messages or data transmitted from the base station. During the active period 200a, the UE can thus monitor the control channel in this way to see if the base station has any data to send. If not, the UE goes to sleep again during the passive period 200b only to wake up for the next active period according to the DRX scheme, and so forth.
A well-known problem in cellular networks is that the performance in communications may be degraded and network capacity may also be reduced, due to interference when the same bandwidth resources are reused in multiple nearby cells, e.g. as illustrated in FIG. 1. For example, when a UE in DRX mode wakes up to monitor control channel during the short active period it DRX scheme, the control channel may be interfered by transmissions in neighbouring cells at that moment such that the UE is not able to receive and properly decode a scheduling message directed to that UE on that control channel, e.g. a Scheduling Assignment on the PDCCH. When no acknowledgement of this message is received from the UE, the base station tries to send the same scheduling message once again during the next opportunity in the DRX scheme since the UE goes to sleep after each active period in the DRX scheme when remaining in the DRX mode.
However, the data which is ready to be sent to the UE will be delayed more and more for each attempt of getting across the scheduling message and the data may become too old for playout, particularly in the case of a delay-sensitive service such as VoIP, resulting in bad QoS as explained above. If the conveying of pending data to the UE fails repeatedly, the communication session will be severely disturbed and may even be interrupted altogether ultimately. This problem is particularly imminent for delay-sensitive services such as VoIP when used by UEs applying DRX at the same time for saving battery since the base station may not be able to get the scheduling message across to notify the UE to start receiving data in time, due to excessive interference on the control channel used.
Another problem associated with the above situation when the base station fails to get across a scheduling message due to interference is that bandwidth resources have been reserved, waiting to be used by the UE provided that it can properly decode the scheduling message. If this process takes too long and the pending data goes out of date and must be discarded, the unused bandwidth resources have been reserved in vain during all this time and will thus be wasted as no-one else can use them meanwhile, which is costly in terms of network capacity.